New equipment advice.

Do a custom sump bigger the better but start looking for cheap used tank something like a 125g would be perfect

Lots of different ways to filter water but sumps usually make the most sense on bigger tanks. Big canisters are another option, pond filters like an ultima II might work(not sure how much they use to backwash) you could build the biggest hmf ever and it would probably work. Just comes down to what your comfortable with and how far you want to go to ensure the best environment for your fishes

Your either going to end up running a plethora of smaller filters, a couple large expensive canisters, or a sump which is most economical.

If your not using your tank you just replaced that would be an ideal gallon size for the sump.

If your not comfortable drilling or having the tank drilled another canister would be the option. I would say use the xp3s since you jave them and add upon that. However then your looking at multiple filters to maintain, the safety of redundancy is nice, but the price and simplicity of a sump is better. Cheaper to run, cheaper to maintain and typically cheaper to build/set up.

I have a 75g im upgrading to a 90g w/ a 40b sump. Dealing with 2 HOB filters and 2 canisters on the 75g is a pain.